UK Revision Notes

P2 – Electricity

Written by

Alfie Frackelton

in

🧩 Electrical Symbols

Learn the common electrical symbols used in circuit diagrams β€” these show components such as resistors, bulbs, switches, cells, and wires.

  • A cell provides energy to the circuit.
  • A battery is two or more cells connected together.
  • A switch opens or closes the circuit.
  • A resistor limits current.
  • A lamp/bulb converts electrical energy to light.
  • A diode allows current to flow in one direction only.
  • A thermistor changes resistance with temperature.
  • A LDR (Light Dependent Resistor) changes resistance with light intensity.

πŸ”„ Current in Circuits

Electric current is the rate of flow of charge around a circuit.
In series circuits, current is the same everywhere.
In parallel circuits, current splits between branches.
Formula:
I = Q / t
(Current = Charge / Time)

⚑ Charge in a Circuit

Charge is measured in coulombs (C) and represents how much electric charge passes through a point.
Formula:
Q = I Γ— t
(Charge = Current Γ— Time)

🧱 Resistance

Resistance is a measure of how much a component opposes the flow of current.

  • High resistance = less current.
  • Measured in ohms (Ξ©).
    Factors affecting resistance:
  • Length of wire
  • Thickness of wire
  • Material
  • Temperature

Formula:
R = V / I
(Resistance = Voltage / Current)

πŸ‡ Voltage, Current and Resistance

V = I Γ— R
This famous formula links voltage (V), current (I) and resistance (R).

πŸ”‹ Voltage in Circuits

In series circuits: voltage is shared between components.
In parallel circuits: voltage across each branch is the same.

πŸ”— Resistors in Circuits

  • Series: Total resistance = R1 + R2 + R3 …
  • Parallel: 1/Rtotal = 1/R1 + 1/R2 + 1/R3 …
    Adding resistors in parallel reduces total resistance.

🌞 Light Dependent Resistor (LDR)

An LDR’s resistance decreases as light intensity increases.
Used in phones and streetlights to detect light levels.

❄️ Thermistor

A thermistor’s resistance decreases as temperature increases.
Used in thermostats, ovens, and car sensors.

πŸ” Alternating Current

AC (Alternating Current) changes direction and voltage repeatedly.
DC (Direct Current) flows in one direction only.
The UK mains supply is:

  • Frequency = 50 Hz
  • Voltage = 230 V

πŸ”Œ Mains Electricity

Three-pin plug wiring:

  • Brown wire: Live
  • Blue wire: Neutral
  • Green/Yellow wire: Earth
    Fuses and circuit breakers protect against excessive current.

⚑ The National Grid

The National Grid transfers electricity from power stations to homes using transformers:

  • Step-up transformers increase voltage to reduce current (less energy lost as heat).
  • Step-down transformers lower voltage for safe domestic use.

πŸ’‘ Energy Transfer in Appliances

Appliances transfer electrical energy to useful forms like heat, light or motion.
Energy transferred = Power Γ— Time
E = P Γ— t

πŸ”‹ Power in Circuits

Power is the rate at which energy is transferred.
Formulas:
P = E / t
P = I Γ— V
P = IΒ² Γ— R

βš™οΈ The Power Trinity

Remember the three power equations:

  1. P = E / t
  2. P = I Γ— V
  3. P = IΒ² Γ— R

Use whichever fits the data you’re given!

πŸ§ͺ Required Practical: Resistance in a Wire

Aim: Investigate how the length of a wire affects its resistance.

  • Set up a circuit with an ammeter and voltmeter.
  • Measure voltage and current for different wire lengths.
  • Calculate resistance using R = V / I.
  • You should find resistance increases with wire length.

πŸ§ͺ Required Practical: IV Characteristics

Aim: Explore how current changes with voltage in components like resistors, bulbs, and diodes.

  • Plot graphs of current (I) against voltage (V).
  • Resistor: straight line (ohmic).
  • Filament bulb: curved (resistance increases when hot).
  • Diode: current flows one way only.

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